The invention relates to a catalyst for catalytic hydrogenation of 1-4 butynediol to 1-4 butanediol.
As is known, the aforementioned hydrogenation reaction of 1-4 butynediol (1) to 1-4 butanediol (III) passes through an intermediate phase in which 1-4 butanediol (II) is produced in accordance with the following equation: ##STR1##
It is also known that the aforementioned hydrogenation reaction is accompanied by a secondary isomerization reaction of 1-4 butenediol (II) to form a by-product comprising .gamma.-hydroxy-butyraldehyde: HOCH.sub.2 CH.sub.2 CH.sub.2 CHO (IV).
This secondary reaction depends on the nature of the catalyst used and on the conditions (temperature and pressure) chosen for hydrogenation of 1-4 butynediol. As is well known, the side-reaction is favoured by the presence of hydrogen, particularly when noble metals are used as catalysts. Consequently, at the end of the catalytic hydrogenation of 1-4 butynediol, the reaction mixture inevitably contains a varying but always substantial amount of .gamma.-hydroxybutyraldehyde. As is known, this results in serious problems when practically quantitative yields are required in the aforementioned reaction, e.g. when applied on an industrial scale.
In order to avoid the presence of .gamma.-hydroxybutyraldehyde in the final product, and in view of the impossibility of preventing the side-reaction of isomerization of 1-4 butenediol, it is necessary to ensure that the aldehyde group of .gamma.-hydroxybutyraldehyde is effectively hydrogenated to a corresponding alcohol group, at the same rate as it is formed.
However, it is known that hydrogenation of an aldehyde group is less easy than saturation of a double or triple bond. Furthermore, catalysts suitable for hydrogenation of unsaturated bonds generally have little or no activity in the hydrogenation of carbonyl groups.
Consequently, in order to obtain acceptable industrial yields the most common prior-art method of catalytic hydrogenation of 1-4 butynediol makes use of a catalyst based on nickel and copper in a ratio of about 4:1.
Owing to the known low activity of the last-mentioned catalyst, the hydrogenation of 1-4 butynediol is brought about at very high pressures, of the order of 200-300 bars, and at temperatures of the order of 140.degree. C. or more, with all the adverse consequences which are well known to the skilled addressee.
In addition the reaction is very slow, which seriously affects the overheads of the aforementioned method of hydrogenation when on an industrial scale.
U.S. Pat. Nos. 2,953,605, 2,967,893, 2,950,326 and 3,449,445 teach the use of Raney type catalyst, based as before on nickel and copper, in one or more stages. This does not substantially reduce the aforementioned technical disadvantages and also, as is known, results in technical complications resulting from a "Ranning catalyst" method and the dangers of using pyrophoric catalysts.
With reference to the use of noble metals as catalysts in a hydrogenation reaction of unsaturated bonds, the basic, unvarying teaching of the hitherto available technical literature is to make predominant use of palladium. However, in view of the high isomerizing power of this metal particularly in the presence of hydrogen, and its known low capacity to hydrogenate aliphatic aldehyde groups to corresponding alcohol groups, it has been suggested that catalytic hydrogenation of 1-4 butynediol should be brought about by using palladium mixed if required with a predetermined percentage of zinc, but only to bring about conversion to 1-4 butenediol, using extremely mild hydrogenation conditions to prevent isomerization of the 1-4 butenediol to .gamma.-hydroxybutyraldehyde, or using catalysts based as before on palladium but suitably poisoned.
The hitherto available technical literature also teaches that ruthenium in one of the most most efficient metals for catalyzing the hydrogenation of aldehyde groups to corresponding alochol groups, but its capacity to catalyze the reduction of unsaturated bonds is so low that it is nowhere recommended for this purpose.
If it is desired to bring about catalytic hydrogenation by using a two-metal catalyst system, one metal favouring the saturation of unsaturated bonds and the other metal favouring the hydrogenation of the aldehyde group (e.g. using nichel and copper or palladium and zinc), the consistently deducible and hitherto confirmed teaching in the technical literature is that the percentage of metal used for hydrogenating the aldehyde group should be appreciably lower than the percentage of the metal chosen as the catalyst for reducing the unsaturated bonds. It is known, however, that hydrogenation based on the aforementioned teaching has not been conveniently or widely applied on an industrial scale, mainly because of the pressure and temperature conditions required for completing the hydrogenation reaction, the low yields and the low reaction rates, which increase the expense of industrial production of the desired saturated compound.
At present 1-4 butanediol is being increasingly used in expanded polyurethane, in polyurethane polymers, in saturated polyesters and in fine chemicals. Its use for these purposes could be greatly increased, both quantitatively and qualitatively, if it could be more cheaply produced.